Automatic telephone system



April 23, 1929. M. L. NELSON 1,7 0,1

AUTOMATIC TELEPHONE SYSTEI Ori ina Filed March 9, 1925 2 Sheets-Sheet l MAI N 52/ Api'il 23, 1929. M. LQ ELSQN 1,710,182

AUTOMATIC TELEPHONE SYSTEI Original Filed larch 9, 1925 2' Sheets-sheaf. 2

lillll EAST (621) .H. if p v i" v munflmmnnm v g v {mum mm A I mhm Ififimw- 1 75+ m L. Nelson Patented Apr. 23, 1929.

UNITED STATES PATENT OFFICE.

MARTIN Ii. NELSON, O]? PAP/K RIDGE, ILLINOIS, .SSIGNOR, BY MESNE ASSIGNMENTS,

TO AUTQIVIATIC ELECTRIC INC., OF CHICAGO, ILLINOIS, A CORPORATION OF DELA- WARE.

AUTOMATIC TELEFHONE Application filed March 9, 1925, Serial No. 14,001. Renewed September 21, 1928.

My .iiwcntion relates in general to auto matic telepln'ine systems, but more purticularly to such autennitic systems include n number of dill'erent oilices or exchanges,

commonly known as multi-ollice systems. A new piece of equipment or apparatus called a (flirector has recentl y been developed for use in systems oil this characte", principally in order to handle the inter-oflice 0 trunking on a more eliicient basis. While the director trunking follows known practice to a large extent, it is much more flexible than methods jfori'i'ierly nfacticed. The introduction of the director opened up certain new lines ol development in trunking and at the same time rendered possible a number 01'. ariations or modifications of known trunking arrangements which could not be used without it. The 'n'esent appli cation deals with certain. improvements which I have made under the foregoing circumstances, and the object, therefore, may be stated to be it novel and improved trunking system for n'iulti-o'llice telephone systems in which directors are used.

The invention has a number of different zspects, or distinct features, which, however, can. best be pointed. out in the course of av more. detailed description of the system in ciimnection with the acc nnpanying drawings.

The drawings,comprising Figures 1 and 2, show diagrammetically the inler-oi'lice trunking between I'ire main offices and one sl'ib-ollice, which are assumed to be part of a large nu'ilti-o'l'lice net-work involving a inuch greater total 11 umber o'l o'llices, perhaps fifty or more altogether. Since the invention has to do only with the trunking, no circuits are shown, but reference is IDEHlO to the follow ing 2i plications which disclose the circuits and apparatus which are suitablefltor use in the system herein described:

1921; T. G. Mn-rm N0. 392.9%, filed June 30, 1920; R. G. Riclm-rdson and M. L. Nelson Scr. No. 538,551, liled Feb. 23, .922; M. L. Nelson Ser. No. 719,719, tiled June 13, 1924.

The drawings may now be described. more in detail, reference being first had. to the lay-out of the lhlidwuy office, Fig. 1. The reference letter A indicates a. subscriber-is station of the usual automatic type. At the Midway e'ilice, with which the substation.

A is associated, the subscribers line terminates in the usual line or trunk selecting switch L. A number of these line switches have common access to a. group of first ofiice selectors, such as the selector FS, which will be referred to hereiiniili'ter for convenience merely as a first selector. The trunk extending from the line switch first selector FS is in two sections, and at the junction of these sections a relay group is provided. This relay group is indicated in the drawing by the reference letters RG and its Function, briefly stated, to enable a director to be associated with the. trunk while a connection is being set up thereover and. to free the director and unitethe two sections of the trunk line for talking purposes after he connection has been es- 'To explain this a little more fully, it may be pointed out that the trunk lines such as the one shown extending to the il'irst selector F8 are divided into groups, the number of trunk lines in each group depending upon the cnl'izicity oi a liuder switch such as F. For example, if the finder switches each have a capacity oil? twentydive lines, then the trunk lines will be divided into gl'QHPS ot twenty-five each. Each of these groups of trunk lines is served by a. grou p of lirectors, the nuniliicr o'l which will depend soi'ncwhat upon .tratlic conditions, but which in any case, will be much less than the number of A. J. Ray Ser. No. 438.985, tiled Jan. 2l,*

trunk lines in a group. The toregoing arrangement is clearly illustrated in the drawing, in which the director D is indicated as having access to the trunk line extending to the first selector FS by means of the finder switch F unda. branch of the trunk line oxierujling downward from the relay group Rt}. This branch is clown indicatedby two parallel linen, accessible to two wipers oi? the finder, because of the fact that it includes both incoming and outgoing cir ends to and from the director.

bank to the away one group at the first selectors, a net ga n of selectors required will,

then the total capacity each group have According to theusual prac .ic

The first selectors,such as FS, otherwise known as ofiice selectors, are used to select trunk lines to the difi'erent o'lliceso't' the system or local trunk lines, and'in case the total number of groups of trunk lines is within the capacity of a single selector, that is, not more than ten, only a single order of switches will be required. In other words, all the trunking outgoing from the Midway otiice could be means of first selectors only. in a large 111llltl-0lll6 system, however, it will usually be the case that considerably more than ten groups of trunk lines are necessary, from which it follows that second selectors will have to be used in addition to the first selectors. The number of groups of second of course, depend upon the nuniber of groups of trunk lines. The drawing indicates a group oi" second selectors accessible from the second level of the first selectors, one second selector, SS, being shown. These second selectors "give access to ten groups of trunk lines, and it it be assumed that the remaining levels of the first selectors are used for trunk lines also, will be nineteen This can be in groups of trunk lines.

creased to any reasonable extent by adding additional groups of second selectors, each group addcd giving an additional capacity of ten groups of trunk lines while taking giving nine groups. 7

tor the Midway office level in either the The local trunk lines may be taken off trorn. a

first selectors or the second selectors, al-

though it is preferable to use a level of the second selectors for this purpose in order to save as many levels of the first selectors as possible for tandem trunking, as will be pointed. out hereinafter. Accordingly, in the present case the trunk lines are taken otl'f roin the fifth l-vei of the second selectors, one of these trui l: lines being indicated. in the drawing as extending to the thousands selector T. Following the usual practi the thousandsselectors have access to ten groups or hundreds selectors, such 7 as the selector H, and the hundreds selectors connectors such as the connector G. The connectors, of course, will have access direct to the subscribers lines which are associated with the Midway otlice.

The trunks out-going from the lllidway otlice to other main otlices include repeaters. e, if there are more than ten trun rs ina grou 1, secondary line switches may be used in orderto give all the selectorswitches common access to all the trunlrs'. This arrangement is indicated in the drawing, where itwill be seen that'the outgoingtrunks from the sixth level of the first selectors such as 1 S terminate accomplished by thousands selector T.

also have access to one or more access to ten groups of in rotary secondary indicate a group of trunk lines incoming to the Midway otlice from the Main otlicc one of which trunk lines is shown as terminating in the incoming selector IS, the banks of which are inultipled with the banks of the The incoming trunks from the sub-exchange Riverside do not terminate in incoming selectors as do the trunk lines troin niain exchanges, but terminate in line switches; This is shown in the drawing where it will be seen that the trunk lines from Riverside to hfllidway come from the bani-cs of secondary line switches such as L in Riverside, and terminate in line switches such as L in Midway. The latter line switches may be inultipled with certain groups of primary line switches, such as t e line switches L and If, associated with substations A and A, respectively.

The'l-ay-out ot the Main ol'lice follows the same general plan that has been explained in connection with the Midway ofiice, except for certain changes in the arrangement oi the otfice and incoming selectors. In view of this,the subscribers lines and the direc' tors are not shown, and th hundreds sclec tors and connectors have been omitted also,

being precisely the sanie as illustrated in such as the relay group RG already described. The firstselector such as FS may have access to certain groups oi direct trunks to other offices, although none are shown in the drawin These selectors inay groups ct second selectors, two such groups being indicated in the drawing, which shows a trunk line extending from the fifth level of the first selector FS and terminating in the second selector SS, and also a trunk line extending from the first level of the selector FS and terminating in the second lcctor SS The thousands selectors for the Main oliice are reached by way of the first level of the latter group of second selectors. The incoming selectors in which the trunk lines from the Midway office terminate, one of thesofselectors being indicated by reference character 1S have their banks connected in niultiple with the banks of the second seleetors SS, while the incoming selectors connected in multiple witlrthe second selectors SS The reason forthis particular arrangement will be pointed out more in detail hereinafter.

From what has been said in the foregoing in connection with the Midway and Main otlices, the layout of the N0rth,South and East oflices, Fig. 2, will be readily understood as to its main features, and it will be urmecessary to point out each of the various switches and specify their function. As in the case of the Main otliee, the subscribers lines and line switches, the directors, the hundreds selectors and connectors have all been omitted, and only the otlice, incoming and thousands selectors are shown. The arrangement of these switches is not precisely like that shown in connection with the Mid.- way and Main oflices, or either of them, but follows the same general plan and the differences will be pointed out in connection with other matters shortly to be described.

The Riverside exchange, although it i given an oliice name or designation likeamain. exchange, is not a. main office but what is known as a sub-office and is subsidiary to the Midway office. The capacity of the River? side sub-oilice is one-thousand lines, the particular thousand used being the fifth thousand of the ten diiferent thousands which are part of the Midway office, this fifth thousand being used for the Riverside sub-office instead of being used in the main exchange.

One of the branch exchange sub-stations is indicated by the reference letter A and the reference letter L indicates theindividual line switch in which the line of this station terminates at the sub-oilice. Lineswitch L", together with a number of other similar line switches, given access to a group of so-called switching selector repeaters, oneof which is indicated by the reference letters SSH. I These switching selector repeaters may be of the type shown in the application of Ray hcreinbeiore referred to. Associated with each switching selector repeater is a secondary line switch such as the line switch L by means of which access is had to trunk lines extend ing to the main or change. On the local level the switching selector repeaters such as SSR have access to hundreds selectors, such as the hundreds selector DH. These hundreds selectors are of the digit-absorbing or dropbaclc' type that is, they release after responding to the first digit." The hereiiiibefore men'tioned application of Martin disr closes a suitable type of drop-backselector for use in this situation. The drop-back selectors, such as DH, have access to ten groups of connectors, such as the connector C, by means of which connections are completed to the local lines in the sub-office.

are energized Midway ex-' Theincoming trunks from the ldidway ofiice terminate in incoming selectors such as IS, the banks of which are multipled with the banks of the drop-back selectors.

Having briefly described the lay-out of the different ollices shown in the drawings, the trunking between these ol'lices will now be considered with a View to pointing out more clearly the Various novel features. It is thought that the trunking can best be explained by considering certain. typical calls and tracing the establishment of the connections as they are set up.

The first call to be explained is a call from the substation A in the Midway office to another substation, A for .lilSl'lli'CQ, in the same otlice. Thetelephone number of sustation A may be assumed to be MlDway There is nothing); new involved in a local connection of this kind, but it is thought best to take up the simplest case first in order that the entire trunking scheme may be thoroughly understood from the beginning. Upon removal of the receivcr at substation A, the line switch L operates in the usual manner to select a trunk line extending to an idle first selector. Assuming that the trunk line extending to the first selector FS the one taken for use, certain relays in. the relay group 11G and complete a holding circuit for the line switch L. At the smile time a starting circuit is closed for startinga finder switch, such as F,which is associated with an idle director. Assuming that the director D is the one which is to be used, finder F operates and connects this director with the trunk line, extending, the callim, line through to the director so that the callinc subscriber can set the register switches therein and establishing a control circuit eK- tending ahead over the trunk'liue to the first selector F5. The calling subscriber. now-operates his dial in accordance with the three letters Mil) of the telephone number of the desired station, and then in accordance with the four digits of such number. The letters MID, which are equivalent to the digits 515, are registered by oiiice registering; equipmcnt'in the director, while the four digits of the number are reo'istm'ed on digit registers in the director. As soon as the office registration completed, a sender switch in the director starts to send out the several series of impulses which are neces sary to control the office lectors in order to trunk the call to the desired oliice, which in this case is the local or or imtinh; oilice. It will he necess ry, thereto 1,-to trunk the call to an idle thousands selector in the Mid ay ofiicc. As is well understood, the sender in the director is controlled in accordance with the setting of, the oiiice registering apparatus. Any particular oiiice registration can be'ma-de to cause the transmission of any phone by a series I the called station.

dcsired number of seriesof impulses, the im- 7 readily changed whenever it is necessary or desirable to do so.

Prooeedingwith the call under consideration, an inspection of the drawings will show that in order to trunk the call to a thousands selector, such asthe selector T 1e ltlidway office, the first selector FS have to be operated to the second level, r which a second selector such as SS have to be operated to the fifth level. The local office code for the Midway office, therefore, is 25, and will be understood now that when the office designation of the number is registered in the director D, the sender in such director immediately proceeds to transmit a series of two impulse to operate the first selector FS, followed of five impulses to operate a or such as the second selector SS, resultii L through first and second selectors to an idle thousands selector such as the selector T. {The director now transmits the four digits of n second scicc the number as they were registered, operat-' ing the selected thousandsselector, a hundreds selectors suchas H, and a connector such as C, to complete the connection to the desired station. On'completing the transmission of all the digits in the called number, the finder switch F isreleased in order to free the director and the t *0 sections of the first selector trunk line are united at the-relay ,roup RG, thus finally completing the connection between the calling and The next case which will be considered is the case of a call from one office to second office which is connected to the first office ver direct trunks. A typical case of this hind is a call from Midway office to Main oflice. Assuming'that the subscriber at tion A in the Midway office desires to connect with a subscriber in the Main office, whose telephone number may be assumed to be Mln 123%, upon removing the receiver at substation A, the calling line-is extended to'an idle first selector such as FS,

and an idle director is taken for use as before. The calling subscriber now operates li-a n "p ,11 1 un 1% ms t 1a 111 LCLOl ounce an n the cesnec e c number and the severailctters and digits called are registered in the director. In this case the office code into which the office portion of thenumber must be translated is ($21, as may be readily ascertained by tracing the trun route from the bank of the first selector'FS in he Mid sends selector T.

in the extension of the trunlc sta- 7 the incoming selector 1S and the second selector SS in the Main oflice, to the thoution of the number is registered, therefore, the director proceeds to transmit series of impulses in accordance with the ofiice'code 621, the first digit of which raises the first selector FS to the siXth level. On finding an idle trunk in this level, which may be assumed to be the one terminating in the secondary line switch L this line switch is operated to select an idle outgoing trunk to the Main office. \Ve may assume that the trunk line shown is selected, whereby it follows that the secondary line switch will stop on such trunk and extend the connection through the repeater R to the incoming selector IS in the Main office. The next series of impulses transmitted bythe sender operates the incoming selector TS to the second level, and such selector then automatically selects an idle second selector. Assuming that the second selector SS is the one taken for use, this selector is then raised to its first level, responsive to the third series of impulses in the office code, and then operates to select a trunk line leading to at thousands select-or. \Ve may assume that the thousands selector T is the one selected. After having transmitted the office code, the director then proceeds to transmit the four registered digits, which are effective to operate the thousands selector T, together with a hundreds selector and a connector (not shown) in order to complete the connection to the desired called station.

Having described briefly the setting up of local calls and inter-ofice calls to offices provided with direct trunks, we may now proceed to a consideration of the more complicated cases which involve tandem trunking. The first call of this general type which will be considered is a call which involves tandem trunking ,over a single route and through one tandem office. A call from the Midway office to the North office ,is a call ofthis kind, and the process of setting up such call will ,now be briefly explained. Assuming that the subscriber at station A desires to call telephone number NORth 123i, and that the same apparatus in the Midway office is used as in the preceding cases, when the subscriber dials the desired number the office portion of the number and the several digits are registered in the usual way. The directors in the Midway office are so cross connected that the office desi nation NOR (602) is not translated, the ofhce code in this case being the same as the numerical equivalent of the letters in the office designation. This, of course, is an unusual case but one which might well occur. Then the office code 602 is transmitted by the director, selectors FS in the Midway office, IS in the Main office, andlS in the North office are As soon as the office por-' ill) til)

t operated, with the result that the connection is extended to an idlethousands selector in the North oflice such as the thousands selector T. It is noted that a secondary line switchL is shown as being inserted between the incoming selector 1S and the repeater R in the Main office. This arrangement is similar to the arrangement shown in connection with the first selector FS and needs no special consideration. The connection having been extended as far as the thousands selector T in the North ollice, it will be readily understood that, as in the preceding cases, the remainder of "the connection is established by operating such thousands selector, a hundreds selector, and a connector in accordrmce with the registered numerical digits of the called teleplmne number.

Before proceeding to the next case, it may i be well to callattention to the otlice code used to trunk calls 'tlOl'rllllG, Midwury ollicc to the South otlicc. As explained in the preceding case, the other: code for the North oliice, considering the directors in the Midway ollice, connnises only three digits, but the otlire code tor the South ollice comprises tour digits, as the trunking in the South ofiice is so arranged that two switches instead of one have to be operated in that olhcc in order to connect with an idle thousands selector, To determine the otl'ice code, he trunking commotion may be tracedfrom he sixth level of the first selector ES in he lrlidway other, by way of the eighth of the incoming selector IS in the otlice, tenth level of the incoming sele tor Thi in the South otlice, and fifth level he second selector SS in the South ol'fice, he l'hraisands selector T he next case which will be considered is a call which involves trunking over a single route through two tandem otl'ices. A typical call of this class is one from a station in the lrlidway oilire to a station in the East office, such call being trunlredthrough the Main and North tandem otlices. l nview of what has been said before, the operations pertor-med in trunking the call to the desired o'tlire vill not need any explanation except to point out the trunk route which is taken,

as this route determines the otlice code for which the directors in Midway must be crossconnected in order to trunk the calls from h'lidway to East over the route such calls are intended to follow. By examining the drawing, it will be seen that a call to East oifice from Midway is first trunked to the -Main ollice by raising a first selector, such selector ES, to the sixth level. From the Main otlice the call is trunl-Zed to the North ollice by operating an incoming selector, such as IS lo the tenth level. The call being now cx't -,-iule l to IS. in the lwt'iltll oil'lce,it is extended to the East. otlice by raising such incoming selector,

routes should become an incoming selector, such as.

to the third level. On this level are ter- I that in the Midway oflice the ollice code for the East ollice is 6033, that is, when any subscriber in Midway oilice calls a number in the East office the otlice designation ELS (623) is translated by the director in use into the otlice code 6 33.

lilaving OXPlztlllBCl several typical cases of tandem trunking over single routes, tandem trunking over alternate routes will next he considered. Calls from Midway to East will be used as an example. As described in the last case, these calls may be routed to the, East olh'ce via the Main and North tandem omens. Another route, however, is equally available, this alternative route bemg by way of the Main. and South tandem otlices. The olliee code 'lorlhe East ottlce at the 'h I idway otlice may Well be (5823 instead of (S033. Assuming that the directors in Midway are crossconnected to translate the East otl'ice designation into the office code (3823, a call to the East oilice would be progressed to its destination. by operating the first selector FS to the sixthv level and an incrnning selector such as IS to the eighth level, resulting in the extension of the call to the South oliice. From this point it is :t'urther extended to a thousands selector in the East otiice by operating an incoming selector such as IS inthe South ollice to the second level and by operating an incoming selector such as IS in the East oflice to the third level. I

The subject of alternative trunking is a very extensive one and no attempt will be made herein to discuss it exhaustively. A few advantages may be pointed out, however, in order that its utility may be somewhat appreciated. One great advantage in the provision of alternative trunk routes between two exchanges is that it prevents service from being entirely interrupted between. such exchanges in case one of the incapable of handling traltic, due possibly to damage to some of the exchange equipment concerned, or to damage to an inter-otlice trunk cable. The latter trouble'is perhaps the most likely to occur. To illustrate this, it may be assumed that all the trallic from the Midway ollicc to the East otlice is normally routed by way of the Main and North tandem otlices. If it be assumed nowthat the inter-office trunk cable for handling traific from the North otiice to the East ollice is damaged and renof the calls will be routed of trunks extending native route is provided, however.

into the office code .ncctmg theremaining threctors in the Midhandlin calls then the cross-connections in the directors in the Midway oflice could be changed so l-Jhat all to the East otlice dered incapable of by way, of the Main and South tanden offices. Such re-routing of the trafiic may,

the group of trunks exto South and the group from South to East, especially during the periods ofpeak load, but during the majority of the time the of course overload tending from Main groups of trunks involved will handle most bf the tratlic and complete interruption of service bet-ween the exchanges concerned is prevented.

As assumed above, all calls from Midway to East may be normally routed by way of the North oflice, the alternative route through the South ofiice being used only in case the route by way of the North office is interrupted for some reason. So long as the alterit is better to normally route part of the tralfic'by way of the North office and part by way of the South oilice, because if this practice is followed, whenscrvice is interrupted over 7 either one of the routes only part of the total amount of traliic will have to be diverted to the remaining route. Thus there will be less danger of theremaining route being disastrously overloaded. In addition a slight increase in efficiency will behad in most cases. It is desirable to have each group of trunks handle calls to as many different exchanges as possible because the pealtload to the different exchanges usually occurs at different times, and if traffic to a number of different exchanges is handled on a single group of trunks the load on the entire group will be better distributed.

The required div sion of traliic is accent rectors in the Midway office so that they will translate the East oilicedesigmiion 6033 and by cross-coir way officeso that they will translate this office designation into the oilice code 6823. If it is desired to divide the trathc equally, one-half the directors in Midway will be cross-connected one way; and one-half the other way. Any desired portion of the traflic can, of course. be diverted to either the North or South otlices by suitable changes in the number of directors that are crossconneetedone way or the other, and since quite a large number of directors are involved as fine a. division as is necessary or desirable can easily be obtained.

The proportion between the calls that are trunked via North and those that are 'trunked via South can bechanged at any are routed via North and half via South, it

' trunks from Main to South, thus evening up groups involved.

from Main to South such IS" in the North oiiice a may happen for some reasonor other that the trailic from resulting in overloading of the group of North, which carry not only the traffic from Main to Nohth but also part of the traiiic from Midway,l\lain, and perhaps other exchanges, to East. Ihorder to alleviate this condi ion a few of the directors in Midway or in Main or in some other exchange which route tra'llic to East by way of North can be re-cross-connected so that they route traflic to East by way of the load on the trunk Numerous other cases will arise in which slight shifts in the trallic routing can be brought about to good advantage in order to relieve temporary conditions of overloading on certain groups of inter-oliice trunks. One case in particular may be pointed out which is interesting inasmuch as it involves two shifts in the traflic routing, the second of which is made necessary or advisable in order to relieve overloading caused by the first. In the case previously referred to. where service over the inter-oilice trunk cable from North to East became interrupted, it was pointed out that all the trafiic to the East otiice from the Midway ofiice (also from Main oiiic-e and otlier ollices'using the trunk route via North) could be routed by way of the South oilice in order to prevent total in-.

terruption of service. It would naturally be expected that this shift in the trailic routing the be relieved somewhat under these circums :nces by re routing part of the trailic from Midway, stain and otheroliices to South otlice by way of the North oiiice. This trallic is all normally handled over the group of trunks but as stated, part of it may be routed by way of the North oliicc. Calls progressed over the new routing would be established by way of the tenth level of incoming or second selectors in the Main olllce the fifth evel of incoming selectors and incoming selectors IS in the South ollice, the banks of which are multiplcd with the banks of the thousands selectors in the South oliice.

Situations will arise where the directors can. be divided along special lines to advantage in order to direct the traflic to distant oflices over alternate routes. For instance there may be some reason for routing a particular class of calls over one route while calls of another class take a di'lferent route. Considering calls from Midway to East again, a part of these calls may be toll or gay station calls, and if the trunk cables involved in the route via North oiiice are of Main to Northhwill increase, a,

heavier gauge or shorter in length than the cables involved in the route via South oilice, it will be desirable to route such toll or pay station calls by way of the North ollice ratlr er than by way of the South otlice. This is easily accomplished. by'so cross-connecting the particular group or groups of directors in the Midway oiiice'which handle the toll and pay station calls so that they will trunk all calls to the East o'llice by way of North otlice, while all or most of the other directors are so cross-connected that they will trunk calls to East otliceby way of the South ol'lice.

The tratlic may also be divided in accordance with the location of the originating station, since it this station is at a some what remote point, the 01 ll should obviously be routedto its destination by way of the best and shortest route. Considering calls originating the hflidway ollice and at the Riverside sub-ollice for example. These calls are all handled by the directors in the lllidway ollice, but those calls'which originate at Riverside obviously should be given the best route because of the added trunk cable involved. If there is any choice between theroutes tlu'ough the North and South ollices, theretore.that is, it one of these routes is better than the other,-the particular group of directors in Midway which handle calls from Riverside should be cross-connected so that they willroute the trallic to the East otlice by way of the best route.

Only a few examples of alternative trunk routes are given in the itorcgoing. An OX- animation of thedrawings will show that a great many more possibilities exist. -Calls from hilain to 'North may be routed direct or by way of South; calls from North to South may be routed direct or by way of East; calls from East to Southmay be routed direct or by way of North calls from East to Main may be routed by way of North,while'calls "from East to Midway may be routed by Way of South, etc. And this isbased on a consideration otonly a very few' ol'lices taken from a large net-work.

hen it is recalled that there will be nu merous other otlices located in ditlerent directions from the ollices shown in the drawing. allot which oflices are inter-connected by trunk lines, the great number of "variations in the trallic routings that are possible will be appreciated. In certain cases it may be possible to progress a call from a given otlice to its destination over as many as halt a dozen alternative routes.

Some further consideration may now be given to the allocation of the groups of outgoing trunlcs to the different levels in the banks of the first and second. ellice sclectors, and to the multipling between the office selectorbanks and the incoming sefrom a level in the lector banks. As mentioned hereinbetore, trunks to heavy tandem ollices are taken first selector banks rather than from a. level in the second, srle banks wherever possible, as ,"hown in the case of the trunks extending .l roiu the Midway otlicc to the lrlain tandem ollice. which come from the sixth. level oi. the first 'se lectors. The reason tor this 's because it reduces the maximum number of digits in the ollice codes that are required to trunl: calls to ditlerent ollices through the tandem o'llice. In the case mention. l, that of the trunks from li'lidwz'iyto Main. the code for trunking a call through the Main 'ollice'to the l last'ollice comprises only four digits, whereas; it the trunl: were taken from the .lcctors such as SS necc. arily comprise directors can be made to tran any desired number of code digits,-it is desirable, of course, to keep the maximum iuuuberthat they have to transmit as low possible in. order to save on the trust cost of the directors. In addition it'ls les' a connection with as low swi ches or links as possible in order to save on the ti 1 cost oi the switches and in order to .H'Illllllllfiit) the liability of trouble andincrease the quality oil. the established talking circuits. The fewer links thatare involved in any connection the less probability there is of sou1ething going wrong while such connection is being set up and the better the circuit will be atter it is established.

The nmltipling between the banks of the .Il'fllll liiiidway to lllain banks oi the :ccorul sothen such code would live digits. lVhile the incoming selectors in the tandem oliices and the olliee or thousands selectors in such tan dem otlices is also arranged with a'view to rconomizing on .the maximum number otl.

digits in the elliee codes i nvolyed. In order to explain. some of the cxpedients,that can be availed of in order to accomplish something in thisdirection the arraugrurents illustrated in the drmviugs may be explained a little more in detail. lc o'usiderin 1 the trunks from. Blidway to hlain, for instance,

one might ordinarily expect that the incoming selectors such as IS in which these trunk lines terminate at the Main otlice would have their banks multipled to the banks of the first selectors such as ES". This would in fact be necessary it the incoming selectors inquestion had to have acto all the different groups of trunks outgoing from the Main oliice. In t io present case, luiwevcn" itis assumed that all of the tra'llic that comes in to the lvlain ottice from the Midway ollice over this group of trunks is intended either for oilice itself or for otlices which are reached Lable to up the ll'lain by wa Y of the )articular zroun of second v o u 4 l selectors including the second selectors SS. Disregardmg tor a moment the tact that point beyond.

' the group of trunks which terminate in inever, level in the banhs ot the second coming selectors such as IS and IS is intended either torthe Main otlicc or for otlices which are reached by way 01' the particular group of second seiectors in Main otlice which includes the second selector SS Such being the case the banks or" the incoming selectors IS tipled to the banirs of the second selectors SS rather than with the banks of the first selectors FS. This arrangement likewise saves a digit in trunking a call through the Main oilice. It will be noted that. the second selectors SS have ac s to the local thousands selectors in the slain oflicc' such as T, so that the incoming selectors IS" and IS", whose banks are multipled with the banks of the second selectors SS", will also have access to such thousands selectors. In the case of the incoming selectors IS however,'access to such thousands selectors 'would be barred were it not for spccialprovisions, and certain situations might arise in which the incoming selectors IS would have to be multipled with the first selectors BS in order to give them access to the thousands selectors in question. In the actual case under consideration, how- 'it is assumed that there is a vacant selectors SS which is not in use toroutgoin ii-units.

This being the case, the corresponding level will be vacant in the banks or incoming sclectccrsIS and can be used for trunking from such incoming selectors to the group of second selectors SS In other words, the group of second. selectors such as SS is made accessible from the first level of the firstselectors such as FS and also from the second level ot the group of incoming selectors which include IS and the neces sary access had while still obtaining the maximum economy in otlice dig'ts.

In view of the foregoing, it will be understood that each group oi incoming selectors in each oflice is multipled with the particular order of switches which will give access to all required points, and that no more orders of switches are included than is absolutely necessary. In the case ofthe trunks from the North oitice to the South office, a still greater economy is possible than in the case previously discussed. t is assumed that in this case the trunks in question handle only tratlic from the North otlice to the South it is not a tandem ofiice for calls from calls from and IS* may be muloffice, which makes possible the multipling of the banks of the incoming selectors such as IS with the banks of the thousands selectors such as T It will be observed from this that while the South office is a tandem ohice tor certain connections, as on a call from Main to East or from Main to South, North to South. In the case of a call from Main to South or from Main to East the mere trunking of the call to the South office does not finally. determine its destination but one or more additional switches have to be operated before a thousands selector in the required otlice can be reached. In the case ot' North to South, howeyer, the trunking of the call to the South otfice finally determines that the call is destined for the South oifice and the incoming selector operates as a thousands selector.

Another non-standard arrangement is shown in the case of the trunks from the South ofiice to the East ofiice, which trunks are accessible from the second level of the first selectors such as FS", and also from the third level of the second selectors such as SS". The reason for multipling these trunks to the banks of the second selectors SS is to enable the incoming selectors such as IS to be multipled with-this group of second selectors rather than with the first selectors such as. FS It is assumed, of course, that the incoming selector such as IS handles trafiic which is intended only for the South cities and other offices reached by way of the second selectors such as SS, with a single exception of calls to the East ofiice, which ofiice is ordinarily'reached by way of thefirst selectors in the South ofiice. By multipling the trunks in question in the banks of the second selectors SS* also, this single exception can be taken care of.

The handling of calls at the sub-oflice at Riverside will next be considered and an explanation will first be made of theprocess of extending a connection from the Midway ofiice to the Riverside sub-ofiice. tors in Midway are cross-connected so that they willtranslate the ofiice designation RIV (216) into the otlice code 25, which is the same as t-he oflice code into which the ofiice designation MID (515) is translated. In establishing a connection extending from Midway to Riverside, therefore, the director in use will operate a first selector such as FS to the second level and a second selector such as thesecond selector SS to the fifth level, resulting in the connection being extended to a thousands selector such as T. The thousands digit of the number is now transmitted, which raises theselected thousands selector to the fifth level, it being rceollccted The direc- Ill this level which extends direct to the River side sub-ollice and there terminates in an in coming selector such as IS. The incomlt will be noted that incstablishing a connection as described above the ollicc code does not actually deternnneto which otlicc the call. is going, this determination being made by the thousands selector in use in the 'l Y T t' -tl' ll I out way ate. -ic mason 01 a m trunks to Riverside from the banksot the thousands selectors in Midway instead of from the banks of a group of otlice selectors in such office may be questioned, since it introduces an additional link in the connection. For instance it these trunk lines were taken from the banks of the second selectors the thousands selectors T would be entirely hy-passed and a shorter route would be obtained. It this were done, however, it would use up a level at the ollice selectors such as SS which might otherwise benecessary tor trunkingto some main otlice, that is, there may be no level available in any group oil. otiice selectors, whereas on the assumption that Riverside is a sub-otlice subsidiary to the Midway otiiee it inherently follows that there will be a level available in the thousands selectors in such ollicc. In addition. it the trunk lines to Riverside were to be selected by ollice selectors in the Midway 'o'llice the incoming selector such as l8 at Riversidewould have to be of the drop-liack variety, as will appear hereinalter. \Vith the arrangement illustrated, these selectors may be ordinary selectors. Another good reason :for the arrangement shown is the fact thatitpermits the incoming selectors such as IS in the Midway ollice to be multipled with the thousands selectors such as T. These incoming selectors handle t-raflic to the Midway ollice andv also to the Riverside sub-o'llice. If the trunks to the Riverside otlice were taken from the banks of some group of ofiice selectors in Midway then these incoming selectors would have to be multipled with such group of oflice selectors. This would iii'itroduce an additional link in all calls incon'iing to the Midway ofiice from the Main oliice and points using the Main o'ltice as a tandem ofiice tor trunking to Midway. which would probably more than outweigh the advantage secured in cutting out a link in trunking calls from the MlClway ottice to the Riverside sub-otlice.

Calls originating at the Riverside subofiice are handled by means of switching selector repeaters as will now be explained. Assuming that the subscriber atstation A desires a connection with subscriber at substation A in the Midway oflice he will remove his receiver and dial the desired number, which may be MlDway 121st. Upon removal ot the receiver the line switch L operates and selectsa trunk line switching selectorextcmling to an idle repeater. i'kssunriug that the switching selector repeater SSR is taken for use the secondary line switch If immediately operates and selects an idle trunk extending to the Midway oll'ice, which may bethe trunk shown in the drawing as terminating in the line switch L. This latterline switch now operates also and selects a trunk leading to an idle first selector in lvlidway. Assuming that the first selector FS is taken tor use a circuit is completed at the. relay group tG by means of which an idle director such as I) is connected to the trunk line by means of a finder suchas F. ()n dialling the first digit of the number the switching selector repeater SSR is raised to the fifth level and at the same time impulses are repeated over the trunk line to the Midway otticc and the digit is registerediu the director I). Since the first digit dialled, the digit 5, has deli nitely determined that the "call is not intended for the Riverside sulrofiicc, the switching selector repeater SSR remains standing opposite the fifth level and does not further respond to the operation of the calling device at the calling station but merely acts as a repeater; The remaining digits of the number, therefore, including the two digits 1 and'6 ol, the ollice designati on and the numerical digits 1, 2, 1, and at, are repeated over the trunk line to the director in use and are registered therein precisely the same as tl'rough the call was originatetfl by a subscriber in the Midway office. It will be understood, therefore, that the director will trunk the call to a thousands selector such as T by translating the office designation into the necessary code, after which it will operate such thousands selector, a hundreds selector such as H, and

a connector such as C, to complete the connection to the desired called station.

A call from station A in the Riverside sub-office which is intended for a local station in the sub-otlice is completed locally as will now be explained. Assuming that the telephone number of the desired local station is 'RIVerside 5678, when the subscriberat station A removes his'reeeivcr the switch ing selector repeater SSR may be taken tor use as before YOSPUIISlVO to the operation of the line switch L".

eration of a line switch such as L associated withthe selected trunk, resulting in the selection of an idle first selector such as FS,

St) I The secondary line 7 switch L" now operates, followed by the opnuled. In uddil'ion lhe incom ng selechirs such as IS are mulliplcd with the newly added thousands selectors instead of will) lhe hundre 'ls solecl'ors. Al? the Midway office he only clum 'e 'lhzili is necessary is lo shilili lhe lrunle; lo Riverside from the fiflh level of lhe thousands selcclors such as '1 io some level of a group of office selccl'ors, the group of office seleelors which includes sic ond selcclor Sb for example. This islhe only change required provided direct frunks are installed helireen ldzrin and Riverside, but if l'he Midi n office is in serve as a lien dem office for frunking to Rivcmide lhen the incoming relecl'ors such as IS will... of course,. have to be mullipled with the HGCfrllll selecl ors such as SS insl'eud of Willi l'he thousands selectors as shown of present. An arrangement of f-h kind may prove lo be economical even in lhe case of a main office; which is being insti led such as the heginnine'. Il results in ii concenfrzilion of all the directors for two offices ill one point and since fhe direr 'ors serve fwire the ordinary number of lines if will he easier l'o secure large and eflicienl: groups. i

Haring described my inrenl'ion. whulv I consider fo he now and desire lo hurc prol'ecl'ed by licl'le rs Pulenl will he poinled out in l he appended claims.

1ml? is claimed is:

l. In {L l'elephone system o firsloffice, :i second office, l'wo intervening tandem offices, directors in the first office arranged lo trunk calls to the second office via the first fizu'adem office and other directors in the firs office orrz'i-nged lo fr ink cells lo the second office via the second tandem office.

2. In :1 telephone system, a first office, :1 second office fwo inhervening ftondein offices, u group of direclors in said first oflicc com mon lo u plurnlile, of lines lermiual'ing in such office, said direclors comprising menus for l'ruuking calls ilrl'un said lines to the required offices, circuit roimec'tions in ccrloin of said directors provii'ling for trunking; calls from said lines lo said second office viii, one e paid lenders offices, and different circuil (fiuli'lQCilOilS in the ren'mining direc lors providing for lrunliing cells from said lines lo sir-id secondoflicc Via the other land 1m office.

In :1. fclephone syslieim afirst office, a

second office, fwo intervening tandem. oflices, directors in the firsl' office for controlling the extension of calls from the first office to (he second office through said f zmdem offices, and means in said directors for lvrunking cells of. u oer-loin class l'o said secinid oflicc 'Vllb lhc faundem office while calls of unolher cluss ore :unlicd to said second office viii said second tandem office.

e. lo :1 telephone system {L- firsl; office a secmid office, two iulcrvenin r fnndem offices, lines terminating in said first office, trunks f ermiunling in said first office. 21 group of. direcfors in said lirsli office for Lrunliing calls from sziid lines to said l'hird office one of said londem offices, and :mother il li of direcl'ors ioisuid first officeon ranged in trunk calls coming-, in over said lruul-Is to send second office by way of the olher inndcm office.

in :1 lundem office, f rst office selcelors, second office selectors accessible lo said lirsl office seleelors, fhouszuuls sel-eclors accessihle lo said second office selecl'ors, incoinine seleofors'having lheir banks i'nullaiplcd with he lklillih of said first office selccliors, ol'her incoming" selectors having" their hunxs multipied with the banks of said secoiul office selecltors and other incoming seleclors halvinc; choir hunks, multipled with the banks of said thousznds selectors.

641i] a tandem office, office, selee-lors fhousunds selectors accessible to said office selectors, incoming selectors having their banks mulf'ipled with the hunks of said office sclecliors and other incoming seleclors haw loo l'heir hunks mullinled with the banks of said llHTdlSfllH-ls selectors.

7. In :1 tandem office, first office selccl'orsq s *cond office .r'elecl'ors accessible to said first office 'selecfors, incomii'ur selectors hearing heir hanks multipled with the banks of said first office selecliors, and other incoming scleclors having their hunks mulliipled with fhe banks of said secoinl office selecf'ors.

8. In a tandem office, office selectors, local selectors accessible to said office selectors, iucoming selectors in two groups coming from two difi'erenf ouf'lyin; offices, respectively :1 multiple between the hunks of said office selcotoru md lahc banks of one group of incoming; selectors, and ii scpzi-iflilc multiple hclwcon lhe banks of said local scle-eliors :uul fhe. hunks of the other group of incoming selecfi'rrs. V

l). in :1 modem office, first office selectors, zi group of? second office seleel'ors ficcesslhle lxo sziid ,lirse office SClfClJl'b} incoming l ors having their hanks inullipled banks of said second office selecfor-= and ii second group of second office selectors necessilole l'o said first office selectors, said second group of second office selectors heing also accessible to said incoming seleclors.

10. In :1 lnndem office, first office selectors, second office selectors accessible to said first office selector-re incoming: seleclors having their banks u'uilfzipled with the banks of said second office selecl'ors, and a group of trunk lines multipled in the banks of all said selc :fors

11. In 2.1 felephone syslcnu :1 nmin office, :1 sub-office swiliohinp sclecl'or repr'alcrs in said sub-office arrziim cd to handle local culls and lo repent l'he (ligril's involved in inaeroffice calls to the main office. and dirccfors in the main office singed to hzmdle calls s 1- Juli originating inthe'main ofiice interoflice calls originating in the sub-ofiiee.

and also all .12. In a sub-otiice, switching selector repeaters accessible :to the subscribers lines, digit absorbing selectors directly accessible to said selector repeaters, and connectors aecessible to said selectors.

In a sub-oiiice, switching selector repeaters accessible to the subscribers lines, digit absorbing selectors directly accessible to said selector repeaters, connectors accessible to said selectors, and incoming lectors having their banks multipled it'll the banks or said selectors.

ll. In a sub-exchange, subscribers staions having telephone numbers each comprising a plurality oi" letters and four digits,

switching selector repeaters in said subollice arranged to switch responsive to the last letter in a local tel phone number, selectors in said sub-o'ltice adapted :to absorb one otthe digits in such telephone number and operate responsive to another, and connectors in the sub-oilice responsive to the lasttwo digits.

15. In. a telephone systenn'a main, oilire, a sub-office, trunks extending "from local see lectors in the main oilice to incoming selectors in said sub otlice, whe eby the capacity of the main otlice is limited to an extent depending on the size of the sub-oliice. lines in the maineilice having numbers which include an office designation derived from the exchange name, and lines in the sub-otlice havingnumbers including a dillerent oll ice designation derived from the name of the subcfiice.

16. In a telephone system, a main otiice, a combinatioumain otlice and sub-o iice, ofiiee selectors in said main ofiice. trunks extending from the banks of said otlice selectors to in: coming; selectors in said combination office, switchingselector repeaters in said combination olliee for handling, local calls and for repeating trunked calls to main oiiice, and directors in said main otlice for han7 tiling calls originating in the manic ice and the calls which are trun'lred to such 'otlice from the combination oilice.

17. In a telephone system, a first ofiice, a second OiliCG, directors in said first office for trunking; calls from said first oiiice to said second oflice in accordance with otlice codes, there being two such codes which may be used, one code comprising more dig L than the other, and circuit conneetions in said directors such that part of the said directors will use one code while other di rectors will use the other code.

18. In a multi ofiice telephone system, a tandem otlice, twotrunl: line groups outgoing from saidtandem oi'lice to two other otlices, respectively, two incoming trunk line groups, automatic switches operable to extend connections from one incoming trunk line group to only one of said outgoing groups and operable to extend connections from the other incoming group of trunk lines to both of said outgoing trunk line groups.

19. In a multi-ofiice telephone system including telephone otlices each designated by the same number of characters as any oneo't' the others, one of said oliices being a .tandem oitice, two outgoingtrunk line groups outgoing from said tandem ()lllf e to two other o said otliccs, respectively, two trunk line groups. incoming: to said tandem oflice, and automatic switching nechanism in said tandem oliice operable to extend connections from one incoming trunk line group to only one oi said outgoing groups and operable to extend connections from the other incoming trunk line group to both of said outgoing trunk line groups.

20. In a multi-otfice telephone system as claimed in claim 19, a third incoming trunk line group, said automatic switching mechanism bein o ierable to extend connections from said 'ilill'Cl incoming trunk line group to telephone lines terminating in said :tandem oilicc. but not to either of said outgoing trunk line groups.

21; In a 'mult-oliice telephone. system including a plurality of main oliices, a subol'lice subsidiary to one oi said main offices, the lines of said sub-otlice having :assigned thereto numbers displacing a block of numbers of said main ollice, each of said olhces having assigned thereto :a designated separate and distinct :trom the designation of every other one of said oliices, and switch- 111g apparatus controlled an accordance with the ollice designation of the sub-oflice when call to the sub-ofii 'e is being made and controlled in accordance with the office designation of the associated 'main office when a call to the main ofiice is being made tor extending; the connection to :the main otlice in both cases.

22. In a multi-otlice telephone system including a plurality of mainotfices, a suboiiice subsidiary 'to one of said main ofii'ces. the lines of said sub-o'ltice having assigned thereto numbers displacing a block ofinumbers of said main ofiice, each of said offices having assigned thereto a designation separate and distinct from the designation of every other one of said otlices, inter-oilicetrunking switches controlled in accordance with the office designations of the respective ofiices, numerical switches controlled in accordance "with the remaining portions of the subscribers numbers, and means for operating the inter-ollice-trunking switches in the same way whether a call. is for said sub-oliice or for :its associated main office for trunking the call to one of the numerical switches in the main ofiice.

2 In a imult-i-ofiice automatic telephone system, three oflices, inter-oflice trunking arrangements so that a connection from one office may he completed to either remaining o'lfice by Way of the other, and means including translating apparatus in the calling oflice for causing calls to be completed either way. I

24-. In. a nmlti-ol'liee automatic telephone system, a calling ofiice, three other ofiices, intcw-o'llice trunking arrangements surl'rthat a call to any one of said other oliices from said calling otlizte may he completed by Way or" the other two of said other o'flices, and means including translating apparatus in the calling oliice for bringing about said routing. v

25. la a ln'ulti otfice auton'iatie telephone system, :t'our offices, ii'lter-oiliee trunking arrangelnents so that callsfrom the first ofiice may he completed directly to the second and to the third o'llices, such that all calls from the first office to the fourth ofiice must pass through either the second or the third called office, such that calls from said first oflice to either the'second or the third office may pass first through the other of the two oflices; and such that calls from the first ofiico to any one of the other oflices may pass through the other two of said offices, and means including translating apparatus in the calling offices for causing any one of said routings to take place.

:ln, witness whereof, I hereunto subscribe my name this (3th day of March, A. 1)., 1

7 MARTIN L. NELSON. 

